High density vertical SRAM cell using bipolar latchup induced by gated diode breakdown

ABSTRACT

Area efficient static memory cells and arrays containing p-n-p-n transistors which can be latched in a bistable on state. Each transistor memory cell includes a gate which is pulse biased during the write operation to latch the cell. Also provided is a CMOS fabrication process to create the cells and arrays.

This patent application is a divisional application of U.S. patentapplication Ser. No. 09/076,487, filed on May 13, 1998, entitled HIGHDENSITY VERTICAL SRAM CELL USING BIPOLAR LATCHUP INDUCED BY GATED DIODEBREAKDOWN.

FIELD OF THE INVENTION

This invention relates generally to non-volatile static memory devices.Particularly, this invention relates to a high density StaticRandom-Access Memory (SRAM) cell taking advantage of the latch-upphenomenon in a Complementary Metal Oxide Semiconductor (CMOS).

BACKGROUND OF THE INVENTION

One known type of static read/write memory cell is a high-density staticrandom access memory (SRAM). A static memory cell is characterized byoperation in one of two mutually-exclusive and self-maintainingoperating states. Each operating state defines one of the two possiblebinary bit values, zero or one. A static memory cell typically has anoutput which reflects the operating state of the memory cell. Such anoutput produces a “high” voltage to indicate a “set” operating state.The memory cell output produces a “low” voltage to indicate a “reset”operating state. A low or reset output voltage usually represents abinary value of zero, while a high or set output voltage represents abinary value of one.

A static memory cell is said to be bistable because it has two stable orself-maintaining operating states, corresponding to two different outputvoltages. Without external stimuli, a static memory cell will operatecontinuously in a single one of its two operating states. It hasinternal feedback to maintain a stable output voltage, corresponding tothe operating state of the memory cell, as long as the memory cellreceives power.

The operation of a static memory cell is in contrast to other types ofmemory cells such as dynamic cells which do not have stable operatingstates. A dynamic memory cell can be programmed to store a voltage whichrepresents one of two binary values, but requires periodic reprogrammingor “refreshing” to maintain this voltage for more than very short timeperiods.

A dynamic memory cell has no internal feedback to maintain a stableoutput voltage. Without refreshing, the output of a dynamic memory cellwill drift toward intermediate or indeterminate voltages, resulting inloss of data. Dynamic memory cells are used in spite of this limitationbecause of the significantly greater packaging densities which can beattained. For instance, a dynamic memory cell can be fabricated with asingle MOSFET transistor, rather than the six transistors typicallyrequired in a static memory cell.

One of the limitations of static memory cells utilizing both n-channeland p-channel devices (CMOS SRAMS) is their exceptionally large cellareas, typically over 100 F², where F is the minimum feature size. Evenusing only n-channel devices, cell size in a compact SRAM design is over50 F². See U.S. Pat. No. 5,486,717. The result is much lower densitiesthan for DRAMs, where the cell size is only 6 or 8 F².

Conventional CMOS SRAM cells essentially consist of a pair ofcross-coupled inverters as the storage flip-flop or latch, and a pair ofpass transistors as the access devices for data transfer into and out ofthe cell. Thus, a total of six Metal Oxide Semiconductor Field EffectTransistors (MOSFETs), or four MOSFETs plus two very high resistanceload devices, are required for implementing a conventional CMOS SRAMcell.

To achieve higher packing densities, several methods are known forreducing the number of devices needed for CMOS SRAM cell implementation,or the number of the devices needed for performing the Read and Writeoperations. However, increased process complexity, extra masks, and highfabrication cost are required and the corresponding product yield is nothigh.

For example, K. Sakui, et al., “A new static memory cell based onreverse base current (RBC) effect of bipolar transistor,” IEEE IEDMTech. Dig., pp. 44-47, December 1988), refers to a Bipolar-CMOS (BICMOS)process in which only two devices are needed for a SRAM cell: onevertical bipolar transistor, and one MOSFET as a pass device. Extraprocessing steps and increased masks are required, along with specialdeep isolation techniques, resulting in high fabrication cost andprocess complexity. Yield of SRAM products utilizing such complexprocesses is usually low compared with the existing CMOS processes.

A problem with CMOS circuits in general is their propensity to“latchup.” Latchup is a phenomenon that establishes a verylow-resistance path between the V_(DD) and V_(SS) power lines, allowinglarge currents to flow through the circuit. This can cause the circuitto cease functioning, or even to destroy itself due to heat damagecaused by high power dissipation.

The susceptibility to latchup arises from the presence of complementaryparasitic bipolar transistor structures, which result from thefabrication of the complementary MOS devices in CMOS structures. Sincethey are in close proximity to one another, the complementary bipolarstructures can interact electrically to form device structures whichbehave like p-n-p-n diodes. In the absence of triggering currents, suchdiodes act as reverse-biased junctions and do not conduct. Suchtriggering currents, however, may be and in practice are established inany one or more of a variety of ways, e.g., terminal overvoltage stress,transient displacement currents, ionizing radiation, or impactionization by hot electrons.

Gregory, B. L., et al., “Latchup in CMOS integrated circuits,” IEEETrans. Nucl. Sci. (USA), Vol. 20, no. 6, p. 293-9, proposes severaltechniques designed to eliminate latchup in future CMOS applications.Other authors, such as Fang, R. C., et al., “Latchup model for theparasitic p-n-p-n path in bulk CMOS,” IEEE Transactions on ElectronDevices, Vol. ED-31, no. 1, pp. 113-20, provide models of the latchupphenomenon in CMOS circuits in an effort to facilitate designoptimizations avoiding latchup.

The present invention takes advantage of the normally undesirablelatchup phenomenon in CMOS circuits to construct a compact static memorycell.

SUMMARY OF THE INVENTION

The present invention provides area efficient static memory cells andmemory arrays by the use of parasitic bipolar transistors which can belatched in a bistable on state with small area transistors. Each bipolartransistor memory cell includes a gate which is pulse biased during thewrite operation to latch the cell. These cells can be realized utilizingCMOS technology to create vertical structures in trenches with a minimumof masking steps and minimal process complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a SRAM cell array with latchup andgated diode according to the present invention.

FIG. 2 depicts a SRAM cell with latchup and gated diode and circuitdiagrams.

FIG. 3 illustrates current-voltage characteristics and avalanchemultiplication in the gated diode structure of the SRAM cell of FIG. 2.

FIG. 4 depicts the blocking, write and latchup states of the SRAM cellof FIG. 2.

FIG. 5 depicts circuit diagrams for the SRAM cell having gated diodeinduced latchup of FIG. 2.

FIG. 6 illustrates a SRAM cell array with interconnect circuitry.

FIG. 7 shows an in-process wafer for producing a SRAM cell array usingoxide isolation on a p+ substrate.

FIG. 8 shows an in-process wafer for producing a SRAM cell array usingan isolated inverted structure on a p-type substrate.

FIG. 9 shows an in-process wafer for producing a non-inverted SRAM cellarray using an additional n-type layer to achieve isolation on a p-typesubstrate.

FIG. 10 shows the wafer of FIG. 9 at a processing step subsequent tothat shown in FIG. 9.

FIG. 11 shows the wafer of FIG. 9 at a processing step subsequent tothat shown in FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The terms wafer or substrate used in the following description includeany semiconductor-based structure having an exposed silicon surface inwhich to form the structure of this invention. Wafer and substrate areto be understood as including doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. Furthermore, whenreference is made to a wafer or substrate in the following description,previous process steps may have been utilized to form regions/junctionsin the base semiconductor structure or foundation. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined by the appendedclaims.

Referring now to the drawings, where like elements are designated bylike reference numerals, an embodiment of the SRAM device array 9 of thepresent invention is shown in FIG. 1. The array 9 is comprised of aplurality of vertical parasitic bipolar transistor pairs on n-type layer34 on p-type silicon substrate 33. Vertical transistor pairs or devices,noted generally 10, are separated from each other by isolation trenches7, 8. Each parasitic bipolar transistor device 10 has dimensions of oneF by one F, and each isolation trench 7, 8 is preferably one F wide.Thus, with the inclusion of transistor to transistor isolation, the areaper programmed device cell is 4 F² (2 F×2 F).

Referring to FIG. 2, a static memory cell, generally designated 5,comprises two complementary bipolar transistors which can latch-up,normally an undesirable characteristic in CMOS but utilized here toconstruct a compact SRAM cell. If vertical structures are used, a 4 F²cell results as also shown in FIG. 2(d). As shown in FIGS. 2 and 3, p+region 17, n-region 16, and p-region 15 comprise a p-n-p bipolartransistor 18; and n+ region 14, p-region 15, and n-region 16 comprisean n-p-n bipolar transistor 19. Thus, each parasitic bipolar transistordevice 10 can be considered as a p-n-p transistor 18 and an n-p-ntransistor 19 connected with the collector of one transistor attached tothe base of the other, and vice versa, as shown in FIGS. 3(b) and 5(b).The center junction (J2) acts as a collector of electrons from (J1) andof holes from (J3). In equilibrium, there is at each junction adepletion region with a built-in potential determined by the impuritydoping profile. When a positive voltage is applied to the anode,junction (J2) will become reverse-biased, while (J1) and (J3) will beforward biased.

There are three sets of interconnects in the device array 9. Row addressline 11 is in connection with lowermost p+ region 17 of each transistordevice 10. Column address line 12 is in connection with the uppermost n+region 14, and write row address line 25 is in connection with gate 13.A high density array is achieved by the use of vertical devices and byplacing gate 13 in the isolation trench 7. Gate 13 gates the central p-njunction (J2) of each transistor structure, as shown in FIGS. 1, 2(d)and (f), and runs within isolation trench 7 on one side of eachparasitic bipolar transistor device 10.

Referring to FIG. 2, showing static memory cell 5, containing gate 13,FIG. 2(b) shows memory cell 5 in the latched condition. FIG. 2(c) showsmemory cell 5 in the blocking (not latched) condition. These conditionsreflect CMOS latchup action, initiated by gated diode currentmultiplication and avalanche breakdown from gate 13, as discussed indetail below. Gate 13 induces latchup in the parasitic bipolartransistor device 10, thus creating one of the two bistable states forthe static memory cell.

If bipolar transistors 18, 19 are off, then the cell will block and notbecome latched until the power supply voltage, V_(DD), becomes veryhigh. However, the cell can be induced to latch up at low power supplyvoltages of a few volts by the application of a pulsed gate bias, thusinducing avalanche multiplication and breakdown in the gated diodestructure in the center p-n junction (J2) as shown in FIGS. 3 and 4.Referring to FIG. 3(b), inversion region 20 in central p-type region 15,and depletion region 21 in central n-type region 16, occur when a pulseof negative voltage is applied to gate 13 and result in gated diodeavalanche breakdown and current multiplication in region 22.

To turn on the device, it is necessary to introduce an external stimuli,e.g., base current by initiating current multiplication in the gateddiode with a pulsed gate bias and higher column voltage. The operationmust be designed such that pulsing yields enough current such that thesum of the common base current gains, α₁ and α₂, of bipolar transistors18, 19 exceeds one. The bias applied to induce latchup is “pulsed” inthe sense that it is only applied to initiate latchup. The cell isstable in the latched condition as a result of the pulse initiatedlatchup, which occurs during the “write” operation as discussed below.

The collector and base currents (I_(C) and I_(B), respectively) and thecommon base forward current transfer ratios or “current gain” α₁ and α₂are shown in FIG. 5(a). From FIG. 5(a), the collector current of then-p-n transistor 19 provides the base drive for the p-n-p transistor 18.Also, the collector current of the p-n-p transistor 18 supplies the basedrive for the n-p-n transistor 19. The base current of p-n-p transistor18, I_(B1), is given by

I _(B1)=(1−α₁)I _(A)

which is supplied by the collector of n-p-n transistor 19. The collectorcurrent of n-p-n transistor 19 with a common base current gain, α₂, isgiven by

I _(C2)=α₂ I _(K)

By equating I_(B1) and I_(C2) with I_(AVALANCHE):

I _(B1) −I _(AVALANCHE) =I _(C2)

Since I_(A)=I_(K), when the collector-base reverse saturation currentsapproach zero (neglecting leakage), then:$I_{A} = \frac{I_{AVALANCHE}}{1 - \left( {\alpha_{1} + \alpha_{2}} \right)}$

which gives the static characteristic of the device up to the breakdownvoltage. I_(AVALANCHE) is small, so I_(A) is small, unless (α₁+α₂)approaches unity; at this point, the denominator of the equationapproaches zero, and latch up will occur.

An illustration of the current multiplication and breakdown voltagesrequired in the gated diode is given in FIG. 3(a). Diode dopings of over10¹⁸/cm³ will result in breakdown voltages (V_(B)) of only a few volts.Region 24 in FIG. 3(a) shows where avalanche multiplication andbreakdown (due to tunneling) occur, in terms of voltage V_(B) and dopantconcentration in the p-n-p-n parasitic bipolar transistor device 10. Thepreferred dopant concentration for the central p-n diode is above 10¹⁷atoms per cm³, with concentrations equal to or above 10¹⁸ atoms per cm³being most preferred.

Referring now to FIG. 6, the array structure of the CMOS SRAM includescolumn decoder 26 and row decoder 27. Data can be read most convenientlyby addressing a row and a column and increasing the power supply voltageacross device 10 to 0.9 V or more at the coincidence of the address. Ifthe cell is latched up, a large current will be sensed between these rowand column lines. If not latched, there will be little extra current.When the cell is not addressed, it can be left in some low voltage statewith V_(DD) around 0.7 V to 0.8 V to reduce power consumption. Read canbe accomplished by lowering the row address voltage.

Write can be accomplished by a coincidence of address in the polysilicongate lines 25 and high column address voltages, to induce carriermultiplication in the gated diode and turn the transistors on strongly.Writing “one” or turning the transistors on and latching up the cell canbe achieved when the cell is in a higher V_(DD) voltage state.

It is most convenient to “write” a row or word as one operation. To doso, the row voltage comes positive to leave some very low value like 0.4V or less across transistors in the row to turn off any transistorswhich are latched up, thus writing “zero” in all cells along the row orword line. Sufficient time is then allowed for any excess base charge inthe latched-up cells to recombine. Following this; “ones” are writteninto selected locations along the word line by a coincidence of row gateline address and selected high column voltages.

In 0.2 micron technology, at moderate forward bias during the readoperation, transistor devices 10 will provide about 100 μA of current.If this is read in 1.6 nanoseconds, then the total signal will be onemillion electrons, which is comparable to the read signal in DRAMs andeasily sensed above any noise. A 4 F² cell will result in an area ofless than 1 cm² for a 128 Mbit SRAM in 0.2 micron technology. If thestandby current in each cell is 10 nanoamperes, then the standby currentwill be 1.28 A and the power dissipation about 1 Watt or 1 Watt/cm²,which is easily dissipated. A ratio of read current to standby currentof 100 μA/0.01 μA can be achieved since the read current is anexponential function of voltage, as shown in FIG. 4. Theseconsiderations can readily be scaled to other size, or minimum featuresize, dimensions.

If planar CMOS peripheral circuits are to be used, the substrate arrayand peripheral circuit doping profiles must be separated. The exactrealization depends on the type of substrate to be used and thetechnology used to isolate the array structures from the substrate. FIG.7 illustrates peripheral area 31, array area 32, epitaxial p-layers 28(EPI) on p+ substrate 29, and oxide isolation layer 30 undercutting thep+ columns in the array area. FIG. 8 illustrates the use of a p-typesubstrate 33 and inversion of the array structure to achieve junctionisolation. FIG. 9 illustrates an array structure which is not inverted,but an additional n-type layer 34 is used to achieve junction isolationon p-type substrate 33. The preferred embodiment described in detailbelow relates to this latter structure, but the techniques described arealso applicable to other structures.

The device array is manufactured through a process described asfollowing, and illustrated by FIGS. 9 through 11 and FIG. 1. First, asilicon substrate 33 is selected as the base for the device array. Thesilicon substrate 33 may be doped or undoped, but a doped p-type waferis preferred. Next, an oxide pad layer 35 is grown or deposited on topof the silicon substrate 33 by means of, for example, thermal oxidationof the silicon substrate 33.

A resist (not shown) and mask (not shown) are applied to coverperipheral circuit area 31 and expose array area 32, andphotolitographic techniques are used to define the array area 32 to beetched out.

An etchant is then applied to define an array window in the oxide pad35. After removing the resist, the remaining oxide pad 35 is then usedas a mask to directionally etch the silicon substrate 33 to a depth ofpreferably about 1 μm. Any suitable directional etching process may beused, including, for example, Reactive Ion Etching (RIE), to form anarray trench in array area 32 of substrate 33.

An oxide layer 36 is then grown or deposited to cover the bare silicon33. Oxide layer 36 is then directionally etched to remove oxide from thetrench bottom, while leaving oxide layer 36 on the vertical side wallsof the array trench. Selective epitaxial silicon is then grown in thearray trench in the following preferred doping profile: 0.1 μm n−, 0.3μm p+, 0.2 μm n−, 0.2 μm p−, 0.2 μm n+, resulting in the cross sectionas shown in FIG. 9.

Oxide pad 35 is then stripped from the surface of the peripheral area31. An oxide pad (not shown) of about 10 nm is then grown atop theexposed n+ epitaxial silicon layer in the array area. Next, a nitridepad 37 is formed by depositing a layer of silicon nitride (Si₃N₄)(“nitride”) by CVD or other means, on top of the pad oxide. The nitridepad 37 is preferably about 60-100 nm thick.

The next step is to define a first set of trenches 8 of the minimumdimension width and space in the column direction. A resist (not shown)and mask (not shown) are applied, and photolithographic techniques areused to define the area to be etched-out. A directional etching processsuch as RIE is used to etch through the pad layers 35 and 37 and intothe silicon to a depth sufficient to expose the buried p+ layer (i.e.,below junction 3 (J3)). The resist is then removed. The set of trenches8 is defined by the sidewalls of the p-n-p-n epitaxial layers. Thetrenches are then filled with silicon oxide by CVD and the surface isplanarized by CMP, stopping on the nitride pad 37.

A second nitride pad layer 37′ is then applied, preferably by CVD, to athickness of about 100 nm. Photolithography is used to define a secondset of trenches 7 orthogonal to the first set of trenches 8. Resist andmask are applied to define the minimum dimension width and space stripesin the row direction. The nitride pad layer and the array layer areetched out by a directional etching process such as RIE to formsidewalls 38 orthogonal to the sidewalls which define the first set oftrenches 8. After etching through the nitride pad to expose alternatesilicon and oxide regions, either a simultaneous silicon/oxide etchantor a sequential etch of oxide and silicon may be used to form trenches 7of uniform depth in the row direction and of sufficient depth to exposethe bottom n-layer as shown in FIG. 10. Etching is continued down to thelevel of the n-layer below junction 4 (J4), and then the resist isremoved.

Oxide layer 40 is then deposited to fill the trenches up to n-type layer(i.e., above J2). Oxide 40 may be planarized by CMP and is preferablydeposited by CVD, and may then be etched back to below J2 , as shown inFIG. 11.

A thin gate oxide 39 is then grown on trench walls 38. A p+ polysiliconlayer 41 is then formed by deposition of doped polysilicon, preferablyby CVD. The thickness of the p+ polysilicon layer 41 is preferable lessthan or equal to about one-third the minimum lithographic dimension.

Referring now to FIG. 1, the next step is to remove excess polysiliconby directional etching of exposed portions of the polysilicon layer 41so that the layer is recessed below the level of junction 1 (J1). Resistand mask are applied to cover alternate trench walls. Polysilicon layer41 is etched to remove exposed polysilicon and leave remainingpolysilicon as gates 13 on one sidewall of each trench as shown in FIG.1.

The device array then undergoes a finishing process. Trenches 7 arefilled with silicon oxide and the surface of the device array isplanarized, by CVD and CMP, respectively, or other suitable processes.Conventional processing methods may then be used to form contact holesand metal wiring to connect gate lines and to equip the device array forperipheral circuits. The final structure of the device array is as shownin FIG. 1.

The process sequence described and illustrated above provides for theformation of minimum dimension programmable devices. It follows thatother structures may also be fabricated, different methods of isolatingthe bipolar transistors, and different methods of forming the p-n-p-ndiodes, such as single dopant and implant techniques, may be realized,by process integration with common process steps.

The above description and drawings illustrate preferred embodimentswhich achieve the objects, features and advantages of the presentinvention. It is not intended that the present invention be limited tothe illustrated embodiments. Any modification of the present inventionwhich comes within the spirit and scope of the following claims shouldbe considered part of the present invention.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a memory cell for storinginformation as one of at least two possible bistable current states, themethod comprising the following steps: providing a semiconductorsubstrate; providing doped silicon regions in the following dopingprofile: p, n, p and n, to form a vertical p-n-p-n diode; forming a p+polysilicon gate adjacent to the central n-p region of said diode; andproviding no more than a first and a second electrical interconnect tosaid diode in addition to said p+ polysilicon gate.
 2. The method ofclaim 1 wherein said substrate is a p-type substrate and said methodfurther comprises the step of forming an insulating material layerbetween the p-n-p-n diode and the substrate.
 3. The method of claim 1,wherein the act of forming said p+ polysilicon gate further comprises:defining a first set of trenches in said doped silicon regions to adepth at least sufficient to expose the lowermost buried p-type layer;defining a second set of trenches orthogonal to said first set oftrenches to a depth at least sufficient to expose the lowermost buriedp-type layer; forming a gate oxide layer on the sidewalls of said firstset of trenches; and defining a p-type polysilicon gate within one ofsaid trenches and adjacent to the central n and p regions of said dopedsilicon.
 4. The semiconductor processing method of claim 3, wherein theact of defining said p-type polysilicon gate further comprises:depositing a p-type polysilicon layer in the first set of trenches;removing a portion of said polysilicon layer while leaving remainingpolysilicon as gate material on one sidewall of each trench of saidfirst set of trenches.
 5. The method of claim 1 wherein said p-n-p-ndiode is inverted.
 6. A semiconductor processing method of forming CMOSstatic access memory within a semiconductor substrate, the methodcomprising the following steps: providing a semiconductor substrate;defining an array trench within said substrate; providing doped siliconregions in said trench in the following doping profile: p, n, p and n;defining a first set of trenches in said doped silicon by directionaletching to a depth at least sufficient to expose the buried p-typelayer, said first set of trenches being no more than about 1 F wide andspaced no more than about 1 F apart, where F is the minimum featuresize; defining a second set of trenches orthogonal to said first set oftrenches by directional etching to a depth at least sufficient to exposethe buried p-type layer, said second sect of trenches being no more thanabout 1 F wide and spaced no more than about 1 F apart; forming a gateoxide layer on sidewalls of said first set of trenches; and defining ap-type polysilicon gate within one of said trenches and adjacent to thecentral n and p regions of said doped silicon.
 7. The semiconductorprocessing method of claim 6, wherein the act of defining a p-typepolysilicon gate further comprises: depositing a p-type polysiliconlayer in the first set of trenches; and removing a portion of saidpolysilicon layer, while leaving remaining polysilicon as gate materialon one sidewall of each trench of said first set of trenches.
 8. Themethod of claim 7, further comprising the steps of depositing oxide tofill the first and second sets of trenches up to past the bottom of thelowermost n-type layer.
 9. The method of claim 6 wherein said substrateis a p-type substrate.
 10. The method of claim 9 wherein said dopingprofile is n−, p+, n−, p− and n+.
 11. The method of claim 9 wherein saiddoping profile is inverted.
 12. The method of claim 9 further comprisingthe step of undercutting said p-type layer to form an evacuated regionand filling said evacuated region with an insulting material.
 13. Asemiconductor processing method of forming CMOS static access memorywithin a semiconductor substrate, the method comprising the followingsteps: providing a semiconductor p-type substrate; defining an arraytrench within said substrate; providing doped silicon in said trench inthe following doping profile: n, p, n, p and n; and n; defining a firstset of trenches in said doped silicon by directional etching to a depthat least sufficient to expose the buried lowermost p-type layer, saidfirst set of trenches being no more than about 1 F wide and spaced nomore than about 1 F apart, where F is the minimum feature size; defininga second set of trenches orthogonal to said first set of trenches bydirectional etching to a depth at least sufficient to expose the buriedn-type layer, said second set of trenches being no more than about 1 Fwide and spaced no more than about 1 F apart; forming a gate oxide layeron sidewalls of said second set of trenches; depositing oxide to fillthe first and second sets of trenches up to past the bottom of thelowermost n-type layer; depositing a p-type polysilicon layer within thesecond set of trenches; and removing said polysilicon layer, whileleaving remaining polysilicon as gate material on one sidewall of eachtrench of said second set of trenches.
 14. A method of forming a circuitfor storing information as one of at least two possible stable currentstates, the method comprising the following steps: providing asemiconductor substrate; providing doped silicon regions to form amulti-region vertical thyristor having at least four regions; forming atleast one polysilicon gate overlying a junction of said multi-regionvertical thyristor; connecting said at least one polysilicon gate to avoltage source for producing latch-up in said multi-region verticalthyristor; and providing no more than a first and a second electricalinterconnect to said thyristor in addition to said polysilicon gate. 15.The method of claim 14 wherein said step of providing doped siliconregions further comprises forming one memory cell.
 16. The method ofclaim 14, wherein said first electrical interconnect is a column addressline in electrical contact with a first end of said thyristor and saidsecond electrical interconnect is a row address line in electricalcontact with a second end of said thyristor opposite to said first end.17. The method of claim 1, wherein said first electrical interconnect isa column address line in electrical contact with a first end of saiddiode and said second electrical interconnect is a row address line inelectrical contact with a second end of said diode opposite to saidfirst end.